1. Field of the Invention
The invention relates to a laser-sintering powder based on polyamide, preferably nylon-12, which comprises PMMI (particles), or PMMA (particles), or PMMI-PMMA copolymer (particles), to a process for preparing this powder, and also to moldings produced by the selective laser-sintering of this powder.
2. Discussion of the Background
In very recent times, a task which is often encountered is the rapid production of prototypes. Selective laser-sintering is a process particularly well suited to rapid prototyping. In this process, polymer powders in a chamber are selectively irradiated briefly with a laser beam, resulting in melting of the particles of powder on which the laser beam falls. The molten particles fuse and solidify again to give a solid mass. Three-dimensional bodies can be produced simply and rapidly by this process, by repeatedly applying fresh layers and irradiating these.
The process of laser-sintering (rapid prototyping) to realize moldings made from pulverulent polymers is described in detail in the patent specifications U.S. Pat. No. 6,136,948 and WO 96/06881 (both DTM Corporation). A wide variety of polymers and copolymers can be used for this application, e.g. polyacetate, polypropylene, polyethylene, ionomers, and polyamide.
Nylon-12 powder (PA 12) has proven particularly successful in industry for laser-sintering to produce moldings, in particular to produce engineering components. The parts manufactured from PA 12 powder meet the high requirements demanded in respect of mechanical loading, and therefore have properties particularly close of those of the mass-production parts subsequently produced by extrusion or injection molding.
A PA 12 powder with a good suitability here has a median particle size (d50) of from 50 to 150 μm, and is obtained as in DE 197 08 946 or else DE 44 21 454, for example. It is preferable here to use a nylon-12 powder with a melting point of from 185 to 189° C., an enthalpy of fusion of 112±17 J/g, and a freezing point of from 138 to 143° C., as described in EP0911 142.
Although the properties of the known polymer powders are indeed good, moldings produced using these powders still have some disadvantages. Particular disadvantages with the polyamide powders currently used are rough surfaces on the moldings, these resulting from the boundary between particles which have undergone melting or incipient melting and the surrounding particles which have not undergone melting. In addition, the formation of extended crystallite structures on cooling of the moldings from polyamide sinter powder is disadvantageous insofar as it is found to give increased shrinkage, or even warpage of the parts. Relatively large components, or components where there is some hindrance to shrinkage, are particularly susceptible to warpage. The very rough surfaces require a coating if parts of suitable quality are to be obtained. In addition, the roughness of the surface causes small structures to merge, making their resolution unsatisfactory.
It was therefore an object of the present invention to provide a laser-sintering powder which can give better dimensional stability and surface quality in the parts produced by means of selective laser-sintering.
Surprisingly, it has now been found that addition of poly(N-methylmethacrylimide) (PMMI), polymethyl methacrylate (PMMA), and/or PMMI-PMMA copolymer to polyamides can produce sinter powders from which, by laser-sintering, it is possible to produce moldings which have markedly better dimensional stability and smoothness than moldings made from conventional sinter powders.